The present invention relates to the modulation of optical signals, and more particularly, to an optical modulation system which includes a self-damped diffractive light modulator.
High-speed optical data modulation systems are used for various applications including optical data storage and communications. These systems require data throughput in the megahertz frequency range. Substantial progress has been made in the development and implementation of microelectro-mechanical (MEMS) based light modulators that operate efficiently at these frequencies. Specifically, Bloom et al. in U.S. Pat. No. 5,311,360 describe an apparatus and method of fabrication for a device for optical beam modulation, known to one skilled in the art as a grating light valve (GLV). Bloom et al. described a similar device in U. S. Pat. No., 5,459,610, with changes in the structure that included: 1) patterned raised areas beneath the ribbons to minimize contact area to obviate stiction between the ribbon and substrate; 2) an alternative device design in which the spacing between ribbons was decreased and alternate ribbons were actuated to produce good contrast; 3) solid supports to fix alternate ribbons; and 4) an alternative device design that produced a blazed grating by rotation of suspended surfaces. Bloom et al. in U.S. Pat. No. 5,677,783 also presented a method for fabricating the device.
According to the prior art, for operation of the GLV device, an attractive electrostatic force is produced by a single polarity voltage difference between the ground plane and the conducting layer atop the ribbon layer. This attractive force changes the heights of the ribbons relative to the substrate. Modulation of the diffracted optical beam is obtained by appropriate choice of the voltage waveform. The voltage needed to actuate a ribbon a certain distance depends on several factors including the tensile stress in the ribbon material and the ribbon length.
It is well known that the ribbon elements of the GLV device possess a resonance frequency which depends primarily on the tensile stress, the density, and length of the ribbons. When a ribbon is actuated or released, it rings at its resonant frequency, which is typically between 1 and 15 MHz. The mechanical response of the ribbon elements is damped by the flow and compression of the layer of gas beneath the ribbons. This phenomenon is referred to as squeeze film damping. It depends on the type of gas present, the pressure, film thickness etc. This damping determines the width of the resonant peak associated with the resonant frequency of the ribbons. As a result of this resonant ringing, the maximum frequency at which the GLV device can be operated is limited, and the diffracted light intensity contains undesirable temporal variations. These temporal variations in a data stream give rise to undesired data errors. Therefore, there is a need for a GLV device having increased operating speed and reduced temporal light intensity variations.
It is an object of the present invention to provide an optical modulation system with a self-damped diffractive light modulator for a beam of light in accordance with an input data stream that is particularly suitable for input data rates greater than 2 MHz.
This object is achieved by a system for modulating a beam of light in accordance with an input data stream having a data rate greater than 2 MHz, comprising:
(a) a source of light for directing light along a predetermined path;
(b) a self-damped diffractive light modulator disposed in the predetermined path and having a plurality of spaced apart self-damped deformable elements being disposed relative to each other and secured at opposite ends and suspended above and movable into a channel containing a gas, and each spaced apart self-damped deformable element having at least one reflective surface;
(c) means responsive to the input data stream for applying forces to each of the spaced apart self-damped deformable elements to cause the spaced apart self-damped deformable elements to deform and move into the channel so that the spaced apart self-damped deformable elements are movable between first and second positions in accordance with the input data stream; and
(d) the self-damped diffractive light modulator modulating the light beam and directing the modulated light to a light utilization device where the modulated light can be recorded or decoded, the spaced apart self-damped deformable elements being sufficiently damped to minimize the introduction of data errors into the modulated light beam.
In accordance with the present invention an optical data modulation system with a self-damped diffractive light modulator suitable for 2 MHz data rates is disclosed. The system represents a significant improvement over existing technology in terms of its data throughput, reliability, and manufacturability. The modulator can readily be optimized at standard ambient conditions, which substantially simplifies fabrication and packaging, and reduces per unit costs.